1. Field of the Invention
The present invention relates to stable adsorber granules which haves high mechanical stability, and also to the use thereof.
2. Brief Description of the Prior Art
Catalyst and adsorber granules, also those based on iron oxides and/or iron oxyhydroxides have already been described. They are used predominantly in continuous processes, in which they are customarily found in tower- and column-like apparatuses through which the medium to be treated flows, where on the outer and inner surfaces of the granules and the accessible pores, the chemical and physical reaction and adsorption processes take place. For this purpose, pulverulent materials cannot be used, because they compress in the direction of flow of the medium and as a result incresse the flow resistance up to blocking of the apparatus.
If an apparatus is cleaned by back-flushing, large amounts of the powder are discharged, are lost, and lead to an intolerable pollution of the wastewater.
The flowing media, however, also exert forces on the granules which can lead to abrasion and/or movement up to vigorous agitation of the granules. As a result the granules impact one another and as a consequence unwanted abrasion results. This leads to loss of catalyst or adsorber material and to contamination of the medium to be treated.
Iron oxide and iron hydroxide-containing adsorption media/reaction media can be used advantageously, for example in the sector of water purification or gas purification. In the case of water purification, this composition is used in filters or adsorber columns through which flow passes horizontally or vertically, or, by addition to the water to be treated for the precipitation of dissolved, suspended or emulsified organic or inorganic phosphorus, arsenic, antimony, sulphur, selenium, tellurium, beryllium and also cyano and heavy metal ions and compounds from, for example, drinking water, service water, process water, industrial wastewater, domestic wastewater, pit water, mineral water, holy water, thermal water and therapeutic water and also aquarium water, garden pond water and agricultural water. Use is also possible in what are termed reactive walls for precipitating the said pollutants from groundwater conduits and leachate water conduits from contaminated sites such as landfills.
In gas cleaning, the composition is used in adsorbers for binding unwanted constituents such as hydrogen sulphide, mercaptans and prussic acid, and also other phosphorus, arsenic, antimony, sulphur, selenium, tellurium and cyano and heavy metal compounds in off-gases. It is also possible to adsorb gases such as HF, HCl, H2S, SOx, NOx.
It is also possible to remove phosphorus, arsenic, antimony, selenium, tellurium and also cyano and heavy metal compounds from used oils and other contaminated organic solvents.
Catalyst granules and adsorber granules based on iron oxides and/or iron oxyhydroxides are also used for the catalysis of chemical reactions in the gas phase or in the liquid phase.
Various processes are also known for removing the trace substances and pollutants from aqueous systems using adsorption media.
For water treatment, preferably continuously operated adsorbers are used which are frequently operated in groups arranged in parallel. To free, for example, drinking water from organic impurities, such adsorbers are charged with activated carbon.
As regards the removal of pollutants such as arsenic from water, iron oxides and/or iron oxyhydroxides are superior to all other known adsorption media. The adsorption of arsenic compounds by iron hydroxide gel has been known for a relatively long time (R. W. Bunsen, A. A. Berthold, 2nd edition, Göttingen, 1837).
DE 4 320 003 A1 describes a process for removing dissolved arsenic by means of solid iron(III) hydroxide.
WO 02/47811 A1 describes a process for producing an iron-containing sorption material with pressure elevation at a temperature below 5° C.
DE 4 320 003 A1 and WO 02/47811 A1 disclose the use of granulated iron hydroxide as adsorber material for a fixed-bed reactor. The granulated iron hydroxide is produced via a freeze-conditioning (freeze drying) of iron hydroxide obtained by neutralization of acidic iron(III) salt solutions at temperatures below minus 5° C. This production process is energetically costly to a great extent and leads to highly salt-polluted wastewaters. Furthermore, as a result of this production process, a very broad particle size spectrum is obtained which also comprises very small particles of low mechanical stability. In the case of use in a fixed-bed reactor, this leads to the grain size spectrum being significantly decreased in the course of the operation due to mechanical abrasion of the particles, which in turn causes finely dispersed particles of loaded or unloaded adsorption medium to be discharged from the reactor. A further disadvantage of these granules is that the adsorption capacity towards arsenic compounds is considerably reduced if the granules lose water, for example owing to a relatively log dry idle time. If the granules are delivered, for example, in the moist state, they have a tendency to stick and are difficult to transport.
In addition the stability of the granules decreases with a high water content which becomes noticeable as an adverse aspect in particular in transport. A high water content also increases the transport costs of the medium. Moist medium offers, in addition, an ideal nutrient base for bacteria which can be washed out into the drinking water during operation. If moist medium comes onto the market, care must be taken that it does not lose moisture in an uncontrolled manner, for instance in drying in air or in the sun, as otherwise the mode of action can be lost and the granule size distribution as a result of shrinkage, and the bulk density, are no longer reliable.
Furthermore, it is known that ageing, for example of amorphous Fe(OH)3 proceeds more rapidly in the aqueous environment than in the dry state. Associated therewith is crystallization and possibly a reduction of the adsorption capacity and stability.
A further disadvantage in this process has proved in practice to be the quality of the granulated material, because the material is comparatively soft and fine-grained. As a result, during backwashing of the filter charged with the material, a considerable material loss occurs. Therefore, at each backwashing, new material must be added, as a result of which the expenditure for carrying out the process is considerably increased. In addition, the water or wastewater to be treated must be relatively clean, since otherwise precipitates of solid constituents of the water on the granules occur due to the filter action and thus the removal of the desired constituents is impaired.
The iron oxide granules or iron oxyhydroxide granules described in WO 92/126630 A1 and WO 02/12663 A1, however, are outstandingly suitable for use in fixed-bed adsorbers. The fines fraction which is possibly abraided off from the. granules by improper mechanical stress of the granules is discharged from the bed from time to time by backwashing together with algae, iron flakes and other fines fractions originating from the groundwater and is collected in a separate vessel end sedimented there. However, in closed water purification containers, for example cartridges or sieve baskets, such a separate extraction of the fines fraction, which could contaminate the treated water, is not possible without further steps. Closed water purification containers comprise the adsorption medium in a bed.
Since the described granules and also the inventive granules can be free from foreign binders, the material is comparatively simple to dispose of after use.
However, the stability of compacted powders is unsatisfactory with relatively long use in adsorbers, and is in need of improvement. Therefore, these formulations come into consideration for use in adsorbers, for example, in particular continuously operated adsorbers, in the purification of water, only with restrictions. In particular in the maintenance or cleaning of the adsorber units by backwashing (see below), such granules, owing to the associated agitation, lose the same large amounts of substance. The backwashing wastewater is highly turbid as a result of the abrasion. This is not acceptable for a plurality of reasons: first, adsorber material is lost which is heavily loaded with impurities after a relatively long service life and is therefore toxicologically harmful. Then, the wastewater stream is polluted with the abraded material which can sediment and thus leads to impairment of the piping systems, and finally the sewage treatment plant is undesirably loaded physically and toxicologically, to name only some reasons.
Frequently, the problem is encountered, especially in regions where well water, mains water or drinking water in general is polluted with arsenic or other heavy metals, that no suitable drinking water treatment plant is available in the vicinity or no suitable system is available which would continuously remove the pollutants.
The company Brita Wasser-Filter-Systeme GmbH has disclosed, for example, cartridges and apparatuses for treating liquids (DE 19 905 601 A1; DE 19 915 829 A1; DE 19 814008 A1, DE 19 615 102 A1, DE 4 304536 A1, U.S. Pat. No. 6,099,728). These apparatuses are very suitable for the complete or partial desalination of drinking water in domestic jugs immediately before use of the drinking water. However, for the easy handleability of the granules in transport, on changing or charging the adsorber tanks, it is necessary that the granules have as low a moisture content as possible, since these otherwise have a tendency to stick together and can no longer flow freely.
The object underlying the present invention, therefore, is to produce improved granules based on iron hydroxide, which granules have a high mechanical stability combined with a high binding capacity for pollutants present in liquids and gases and which have flowability and low water content and which achieve sufficient mechanical stability even without organic binders or inorganic foreign binders.